The present invention pertains to the technical field of physical sciences and technology. More specifically it involves technologies of vacuum technique, gas dynamics and charged particle beams.
There are instruments which require the transfer of particle beams from one chamber of low pressure (or vacuum) into a chamber with substantially higher pressure. The maintenance of a pressure difference between two chambers communicating with each other via a free aperture is achieved with the well known art of differential pumping. According to this art, one of the chambers is pumped out while the other is maintained at constant pressure by supplying gas to compensate the amount of gas leaking through the aperture. A steady state situation of gas flow and a steady pressure difference is achieved between the two chambers. This constitutes a single stage differential pumping and it may be repeated to form two, three and so on stages of differential pumping.
Sharp pressure gradients develop along the axis of the aperture which limits the gas flow. The aperture resists the flow of gas like a resistor opposes the electrical current In an electrical circuit. Gas flows from the chamber of high pressure into the chamber of low pressure and a supersonic jet develops downstream shortly past the aperture plane. This aperture has been referred to as pressure limiting aperture (PLA) in the literature (references 1,2) and has been studied by the science of gas dynamics. The amount of gas flow and the length of jet depends on the geometry of the aperture, the nature and temperature of gas and the pressures used in the two chambers. These properties determine the performance and cost of the system and impose some practical limits. For example, in order to achieve high pressure differentials with large apertures, large pumping capacities are required. Furthermore, the supersonic gas jet can extend over a long path which can interfere with the performance of certain instruments. The present invention discloses a radically new method and apparatus that it both overcomes the disadvantages of prior art and it also constitutes an alternative approach to achieve an equivalent result to the conventional pressure limiting aperture.
According to the present invention, a substantial pressure difference between chambers communicating via a PLA can be achieved by forcing the gas from the chamber of low pressure to flow towards the chamber of high pressure using the pumping action of the Venturi principle. According to the Venturi principle, a gas stream creates its own pumping action by entraining gas molecules of the surrounding gas. The inventive step of the present invention lies in the introduction of an annular supersonic jet surrounding the PLA and flowing towards the high pressure chamber. The gas of the annular jet is supplied by a third chamber of a higher pressure than the pressure of the chamber into which it flows. In the steady state situation, pressure gradients are again established between the two sides of the PLA.
The Venturi principle is well known and has been used in the construction of vacuum pumps (references 3,4,5). The present invention integrates this principle with the design of a pressure limiting aperture to be used in whatever instruments it is required. One example of such an instrument, on which it can be used, is the environmental scanning electron microscope (ESEM) whereby an electron beam is transferred from chambers of high vacuum into chambers of progressively higher pressure.
This invention relates to a novel device for achieving a pressure difference between two chambers, namely, between first and second chamber, which communicate via an aperture (the PLA). A third chamber is added, which is maintained at substantially higher pressure than the first chamber with which it communicates via an annular orifice surrounding the PLA. Gas flows from the third chamber into the first chamber through the annular orifice, and an annular supersonic jet develops in the first chamber. The core of the annular jet creates a Venturi pumping action by entraining any gas molecules entering In it. The core of the annular jet communicates via the PLA with said second chamber which, as a result, is evacuated by the pumping action of the annular jet. Gas is removed from the first chamber by a vacuum pump at a rate equal to the rate of gas supply by the annular jet plus the relative rate of gas supply by the PLA. In the steady state situation, a substantial pressure difference is established between the first and second chamber due to the pumping action of the annular jet.
There is an interdependence among the pressure levels of first, second and third chamber, which depends on the design characteristics of the system. The ultimate pressure differential that is achieved by the present invention critically depends on the geometry, pressures, temperatures, nature of gases as well as the materials and surfaces used in the system. The optimum system is also dependent on the practical requirements of the purpose (i.e. application) for which it is used.
Although the basic features of the invention have been described above, it will be appreciated that numerous variations, modifications, or alterations may be effected to certain parameters of the system without departing from the spirit or scope of the invention. One such parameter is the geometry of the flow, which will be examined in more detail in the description that follows. The gas flow characteristics can be modified by the presence of certain objects placed in first chamber under certain conditions. An object will not substantially affect the gas flow when it is placed at sufficiently long distance from the PLA. However, an object will modify the geometry of the flow and hence the gas flow properties when it is placed within the range of action of the annular jet. Out of numerous cases, two cases involving variations of geometrical flow are described below.
It is a another form of the present invention to insert objects that modify the gas flow in order to achieve a specific result. When an object is placed within the range of action of the annular jet, a higher pressure is generally exerted on the specimen close to the axis of the jet than the background pressure of first chamber. This higher pressure is due to the dynamic pressure exerted by the kinetic energy of the jet and it can be regulated by the geometry of the system. This higher pressure can be used as an advantage to achieve a higher pressure differential between first and second chamber. In order to equalise the background pressure of the first chamber to the higher pressure at the axis of the jet, a baffle (object) is inserted and fixed in the first chamber. This baffle comprises a surface with an orifice placed co-axially with the PLA. The annular jet impinges on the baffle and the reflected gas flows over the baffle wherefrom it is removed with a pump.
It is a further form of the present invention to create maximum gas concentration only along the axis of the system in the first chamber, namely, the creation of a high intensity molecular beam. A molecular beam formed inside the first chamber constitutes the opposite effect achieved with the baffle outlined above. A molecular beam is formed by replacing the baffle with a skimmer such as is known and used in molecular beam technology. A skimmer has the property of separating, or xe2x80x9cskimmingxe2x80x9d, the outer region of a jet while allowing the passage of the core of a jet undisturbed through the orifice of the skimmer. In this case, pumping is required on both sides of the skimmer, namely, first to remove the skimmed gas from the annular jet on one side of the skimmer and second to remove the gas of the molecular beam after the latter strikes a specimen under examination. In this configuration of the invention, the pressure differential is formed between second chamber and the core of a molecular beam in first chamber. This allows the transfer of an electron or ion beam from the low pressure of second chamber into the first chamber along and inside a molecular beam, so that both the charged particle beam and the molecular beam are concurrent and co-interact with the specimen surface under examination.
The flow characteristics can be further modified by varying the shape of the annular orifice, but this invention is not limited by a particular choice of this geometry.
Another parameter that modifies the performance of the present device is the temperature of various gases and surfaces of the flow, especially the temperature of the input gas in said third chamber. The range of pressure levels and pressure differentials obtained is a function of the temperature. It will be appreciated that no particular choice of this parameter deviates from the scope and spirit of this invention.
The present invention is based on a well known physical principle which is used in air operated Venturi and in diffusion vacuum pumps. However, it can be ascertained by anyone skilled in vacuum technology that both the scope and design of the present invention is different from all previous art. The physical principle of this invention has not been previously used in the mode and scope disclosed herewith. Furthermore, the present invention adds a new design of pump with a specific range of application to vacuum technology itself. In addition, the configuration of the present invention can be used to design and construct Venturi type vacuum pumps, wherein said third chamber is connected with a positive air pressure source, said second chamber forms the suction (inlet) of the pump and said first chamber is open to ambient atmosphere.
A corollary disclosure of the present invention reveals means for forming a dense molecular beam, in general. The present method differs from conventional sources where the jet is formed in a single central nozzle and subsequently skimmed. The present system starts with an annular jet, instead, formed around a central spindle with or without a hole. This system is equivalent, or acts in a similar manner, to a garden hose sprinkler whereby a water jet can be xe2x80x9cfocussedxe2x80x9d or xe2x80x9cspreadxe2x80x9d by shifting the spindle along the axis of the sprinkler nozzle.
These and further objectives of the invention will be apparent from the following description of the invention as illustrated in the accompanying three Figures.